科研之友
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摘要
Fe2+- and Mn2+-rich tourmalines were used to test whether Fe2+ and Mn2+ substitute on the Z site of tourmaline to a detectable degree. Fe-rich tourmaline from a pegmatite from Lower Austria was characterized by crystal-structure refinement, chemical analyses, and Mossbauer and optical spectroscopy. The sample has large amounts of Fe2+ (similar to 2.3 apfu), and substantial amounts of Fe3+ (similar to 1.0 apfu). On basis of the collected data, the structural refinement and the spectroscopic data, an initial formula was determined by assigning the entire amount of Fe3+ (no delocalized electrons) and Ti4+ to the Z site and the amount of Fe2+ and Fe3+ from delocalized electrons to the Y-Z ED doublet (delocalized electrons between Y-Z and Y-Y): (X)(Na0.9Ca0.1)(Y)(Fe2.02+Al0.4Mn0.32+Fe0.23+) (Z)(Al4.8Fe0.83+Fe0.22+Ti0.14+)(T)(Si5.9Al0.1)O-18(BO3)(3)(V)(OH)(3) (W)[O0.5F0.3(OH)(0.2)] with a = 16.039(1) and c = 7.254(1) angstrom. This formula is consistent with lack of Fe2+ at the Z site, apart from that occupancy connected with delocalization of a hopping electron. %26lt;br%26gt;The formula was further modified by considering two ED doublets to yield: (X)(Na0.9Ca0.1) (Y)(Fe1.82+Al0.5Mn0.32+Fe0.33+)(Z)(Al4.8Fe0.73+Fe0.42+Ti0.14+)(T)(Si5.9Al0.1)O-18 (BO3)(3) (V)(OH)(3) (W)[O0.5F0.3(OH)(0.2)]. This formula requires some Fe2+ (similar to 0.3 apfu) at the Z site, apart from that connected with delocalization of a hopping electron. Optical spectra were recorded from this sample as well as from two other Fe2+-rich tourmalines to determine if there is any evidence for Fe2+ at Y and Z sites. If Fe2+ were to occupy two different 6-coordinated sites in significant amounts and if these polyhedra have different geometries or metal-oxygen distances, bands from each site should be observed. However, even in high-quality spectra we see no evidence for such a doubling of the bands. We conclude that there is no ultimate proof for Fe2+ at the Z site, apart from that occupancy connected with delocalization of hopping electrons involving Fe cations at the Y and Z sites. %26lt;br%26gt;A very Mn-rich tourmaline from a pegmatite on Elba Island, Italy, was characterized by crystal-structure determination, chemical analyses, and optical spectroscopy. The optimized structural formula is (X)(Na-0.6 square(0.4)) (Y)(Mn1.32+Al1.2Li0.5) Al-Z(6) TSi6O18 (BO3)(3) (V)(OH)(3) (W)[F0.5O0.5], with a = 15.951(2) and c = 7.138(1) angstrom. Within a 3 sigma error there is no evidence for Mn occupancy at the Z site by refinement of Al %26lt;-%26gt; Mn, and, thus, no final proof for Mn2+ at the Z site, either. %26lt;br%26gt;Oxidation of these tourmalines at 700-750 degrees C and 1 bar for 10-72 h converted Fe2+ to Fe3+ and Mn2+ to Mn3+ with concomitant exchange with Al of the Z site. The refined Fe-Z content in the Fe-rich tourmaline increased by similar to 40% relative to its initial occupancy. The refined Fe-Y content was smaller and the %26lt; Y-O %26gt; distance was significantly reduced relative to the unoxidized sample. A similar effect was observed for the oxidized Mn2+-rich tourmaline. Simultaneously, and F were expelled from both samples as indicated by structural refinements, and H expulsion was indicated by infrared spectroscopy. The final species after oxidizing the Fe2+-rich tourmaline is buergerite. Its color had changed from blackish to brown-red. After oxidizing the Mn2+-rich tourmaline, the previously dark yellow sample was very dark brown-red, as expected for the oxidation of Mn2+ to Mn3+. The unit-cell parameter a decreased during oxidation whereas the c parameter showed a slight increase.
- 出版日期2012-9
